The present invention relates generally to radio antennas, and more particularly to an antenna for radio communication devices.
The size of wireless handheld communication devices, such as cellular telephones, is being driven by the marketplace towards smaller and smaller sizes. Consumer and user demand has continued to push a dramatic reduction in the size of communication devices. As these devices become less bulky, users face an increasing number of options for carrying and using the device. For example, early cellular telephones were so large and heavy that they could not comfortably be carried in a pocket or on a belt holster. It is envisioned that future portable devices will be thin and light enough to be easily carried in a shirt pocket or worn like a badge. However, the operation of such small devices in close proximity to a user affects the radio frequency transmission and reception qualities of the device. Moreover, such devices will still need to properly operate over multiple frequency bands and with various existing cellular system operating modes, such as TDMA, CDMA, GSM and even future operating systems.
Prior art antenna systems have utilized an extendable antenna shaft and various passive couplings to coils and capacitances to achieve an improved impedance match for the communication device to properly operate at various frequencies. Unfortunately, these systems are still relatively bulky when considering a phone that will be reduced to a credit-card size. In particular, placing a loading coil around a shaft while keeping the shaft mechanically rugged for a small phone would be difficult to achieve. Moreover, due to the thinness of a credit-card sized phone (about 5 mm), any extendable antenna shaft would necessarily be placed very close to the skin of a user during operation, compromising antenna efficiency and specific absorption rate (SAR) of the RF signal. Other prior art systems, such as planar inverted F antennas (PIFAs) have been used to provide dual band operation, but these systems do not provide the complete functionality required for smaller telephones. Conventional monopole antennas will not satisfy efficiency and SAR requirements in a shirt pocket position (the probable preferred user position for a thin-profile phone). However, an extended monopole antenna would provide more satisfactory performance when used by a user""s head, due to the maximized distance between the radiating currents and the user""s head and hand. Therefore, a conventional monopole will not satisfy the performance and safety requirements of cellular telephone operation when located at both a user""s head and in a user""s shirt pocket.
The need for enhanced operability of future communication devices along with the drive to smaller sizes results in conflicting technical requirements for the antenna. Impedance match, radiation efficiency, and minimum RF exposure to the user dictates different antenna solutions and implementation schemes for different operating modes. Further, the location of the device on a user during operation changes these parameters. For example, the smallness of the device may result in the entire device being shielded within a user""s hand reducing radiation efficiency when not using an extendable antenna. In addition, the device must meet more stringent mechanical requirements in a manner that is sufficiently rugged. In particular, smaller devices are susceptible to flex stresses that can occur when carrying the device in a wallet, purse, pants pocket or shirt pocket during even mild user activities such as bending, walking, and sitting.
Accordingly, what is needed is an antenna system for a radio communication device that is mechanically rugged. It would also be of benefit, to provide an antenna system that is operable in very different antenna modes required for very different antenna states. Moreover, it would be an advantage to provide proper operation of the device in different operating positions with minimum complexity.